Semiconductor nanoparticles and method of producing semiconductor nanoparticles

ABSTRACT

A semiconductor nanoparticle includes a core and a shell covering a surface of the core. The shell has a larger bandgap energy than the core and is in heterojunction with the core. The semiconductor nanoparticle emits light when irradiated with light. The core is made of a semiconductor that contains M1, M2, and Z. M1 is at least one element selected from the group consisting of Ag, Cu, and Au. M2 is at least one element selected from the group consisting of Al, Ga, In and Tl. Z is at least one element selected from the group consisting of S, Se, and Te. The shell is made of a semiconductor that consists essentially of a Group 13 element and a Group 16 element.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.16/732,877, filed Jan. 2, 2020, which is a divisional application ofU.S. patent application Ser. No. 15/459,767, filed Mar. 15, 2017, whichclaims the benefit of Japanese Patent Application Nos. 2016-055299,2016-177631, and 2017-037487, filed on Mar. 18, 2016, Sep. 12, 2016, andFeb. 28, 2017, respectively. The entire disclosures of all are herebyincorporated by reference.

BACKGROUND Technical Field

The present disclosure relates to semiconductor nanoparticles, a methodof producing the semiconductor nanoparticles, and a light-emittingdevice using the semiconductor nanoparticles.

Description of Related Art

White light-emitting devices, which are used as backlights of a liquidcrystal display devices and the like and which utilize photoluminescenceemission from quantum dots (also called “semiconductor quantum dots”),have been proposed. Fine particles of semiconductor with a particle sizeof 10 nm or less, for example, are known to exhibit a quantum sizeeffect. Such nanoparticles are called the quantum dots. The quantum sizeeffect is a phenomenon where a valence band and a conduction band, eachof which is regarded as continuous in bulk particles, become discretewhen the particle size is on the nanoscale, whereby a bandgap energy isvaried in accordance with their particle size.

Quantum dots absorb light and emit light corresponding to the bandgapenergy thereof. Thus, quantum dots can be employed as a wavelengthconversion material in light-emitting devices. Light-emitting devicesusing quantum dots are proposed, for example, in Japanese UnexaminedPatent Application Publication No. 2012-212862 and Japanese UnexaminedPatent Application Publication No. 2010-177656. More specifically, aportion of the light emitted from a light-emitting diode (LED) chip isabsorbed by quantum dots, and photoluminescence of another color isemitted from the quantum dots. The photoluminescence emitted from thequantum dots and the light from the LED chip not absorbed by the quantumdots are mixed to produce white light. In these patent applicationdocuments, use of quantum dots made of a Group 12-Group 16 material,such as CdSe or CdTe, or a Group 14-Group 16 material, such as PbS orPbSe, is proposed. In International Patent Application Publication No.WO 2014/129067 A, a wavelength conversion film in which core-shellsemiconductor quantum dots that does not contain Cd or Pb in view oftoxicity of these elements is proposed. The formation of such acore-shell structure is also described in Chemical, Communications.2010, vol. 46, pp 2082-2084.

One of the advantages of using quantum dots in light-emitting devices isthat light with a wavelength corresponding to a bandgap of the quantumdots can have a peak with a relatively narrow full width at halfmaximum. Of the quantum dots proposed as the wavelength conversionmaterial, quantum dots made of a binary semiconductor, typified by Group12-14 semiconductors, such as CdSe, is confirmed to emit the light withthe wavelength corresponding to the bandgap of the quantum dots, i.e.,band-edge emission. Meanwhile, ternary quantum dots, in particular,Group 11-13-16 quantum dots have not been confirmed to exhibit theband-edge emission.

The light emission from the Group 11-13-16 quantum dots is caused by thedefect levels at the surface or inside of the particles, or by thedonor-acceptor-pair recombination, so that such light has a broadphotoluminescence peak with a wide full width at half maximum and a longphotoluminescence lifetime. Such light emission is not suitable forlight-emitting devices, particularly, used in liquid crystal displaydevice. This is because a light-emitting device used in a liquid crystaldisplay device is required to emit light with a narrow full width athalf maximum that has a peak wavelength corresponding to each of threeprimary colors (i.e., RGB) in order to ensure high colorreproducibility. For this reason, practical application of the ternaryquantum dots has not been prompted despite its less toxic composition.

SUMMARY

Therefore, one object of certain embodiments of the present disclosureis to provide semiconductor nanoparticles that are configured to produceband-edge emission from ternary or quaternary quantum dots with a lesstoxic composition, and a method of producing the semiconductornanoparticles.

According to certain embodiments of the disclosure, a semiconductornanoparticle includes a core and a shell covering a surface of the core.The shell has a bandgap energy larger than a bandgap energy of the coreand is in heterojunction with the core. The semiconductor nanoparticleis adapted to emit light upon being irradiated with light. The core ismade of a semiconductor that contains M¹, M², and Z. M¹ is at least oneelement selected from the group consisting of Ag, Cu, and Au. M² is atleast one element selected from the group consisting of Al, Ga, In, andTl. Z is at least one element selected from the group consisting of S,Se, and Te. The shell is a semiconductor that consists essentially of aGroup 13 element and a Group 16 element.

Furthermore, according to other certain embodiments of the disclosure, amethod of producing semiconductor nanoparticles includes:

providing a dispersion in which primary semiconductor nanoparticles aredispersed into a solvent, each of the primary semiconductornanoparticles being made of a semiconductor that contains M¹, M², and Z,where M¹ is at least one element selected from the group consisting ofAg, Cu, and Au, M² is at least one element selected from the groupconsisting of Al, Ga, In, and Tl, and Z is at least one element selectedfrom the group consisting of S, Se, and Te; and

adding, to the dispersion, a compound containing a Group 13 element andan elemental substance of a Group 16 element or a compound containing aGroup 16 element, to form a semiconductor layer consisting essentiallyof the Group 13 element and the Group 16 element on the surface of eachof the primary semiconductor nanoparticles.

In the semiconductor nanoparticles according to certain embodiments ofthe present disclosure, the surface of the core made of a ternarysemiconductor is covered with the shell made of the semiconductorcontaining the Group 13 element and the Group 16 element, the shellhaving a bandgap energy larger than that of the semiconductor formingthe core. With this arrangement, photoluminescence emission with a shortphotoluminescence lifetime, i.e., the band-edge emission, which cannotbe obtained by conventional ternary quantum dots, is obtained. In thesemiconductor nanoparticles, each of the core and shell can havecomposition that contains neither Cd nor Pb, which are highly toxic.Thus, the semiconductor nanoparticles can be applied to products thatare prohibited from using Cd or the like. Therefore, such semiconductornanoparticles are suitable for use as a wavelength conversion materialfor light-emitting devices used in the liquid crystal display device orthe like or as a biomolecule marker. Furthermore, with the method ofproducing semiconductor nanoparticles as described above, the ternarysemiconductor nanoparticles that exhibit band-edge emission can berelatively easily produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an X-ray diffraction (XRD) pattern of primary semiconductornanoparticles (cores) produced in Example 1.

FIG. 2 shows a photoluminescence spectrum of core-shell semiconductornanoparticles produced in Example 1.

FIG. 3 shows optical absorption and excitation spectra of the core-shellsemiconductor nanoparticles produced in Example 1.

FIG. 4 shows an emission spectrum of the primary semiconductornanoparticles (cores) produced in Example 1.

FIG. 5 shows an optical absorption spectrum of the primary semiconductornanoparticles (cores) produced in Example 1.

FIG. 6 shows a photoluminescence spectrum of core-shell semiconductornanoparticles produced in Example 2.

FIG. 7 shows optical absorption and excitation spectra of the core-shellsemiconductor nanoparticles produced in Example 2.

FIG. 8 shows a photoluminescence spectrum of core-shell semiconductornanoparticles produced in Example 3.

FIG. 9 shows optical absorption and excitation spectra of the core-shellsemiconductor nanoparticles produced in Example 3.

FIG. 10 shows a photoluminescence spectrum of semiconductornanoparticles produced in Comparative Example 1.

FIG. 11 shows an optical absorption spectrum of the semiconductornanoparticles produced in Comparative Example 1.

FIG. 12 shows photoluminescence spectra of semiconductor nanoparticlesproduced in Examples 4A to 4D.

FIG. 13 shows photoluminescence spectra of semiconductor nanoparticlesproduced in Examples 5A to 5F.

FIG. 14 shows a graph showing the relationship between the temperatureholding time and the peak intensity of the band-edge emission of thesemiconductor nanoparticles produced in Examples 4A to 4D and thesemiconductor nanoparticles produced in Examples 5A to 5F.

FIG. 15 shows a graph showing the relationship between the temperatureholding time and the peak wavelength of the band-edge emission of thesemiconductor nanoparticles produced in Examples 4B to 4D and thesemiconductor nanoparticles produced in Examples 5B to 5F.

FIG. 16 shows a high-resolution transmission electron microscope (HRTEM)image of the semiconductor nanoparticles produced in Example 5F.

FIG. 17 shows a high-angle annular dark field (HAADF) image of thesemiconductor nanoparticles produced in Example 5F.

FIG. 18 shows a photoluminescence spectrum of core-shell semiconductornanoparticles produced in Example 6.

FIG. 19 shows an absorption spectrum of the core-shell semiconductornanoparticles produced in Example 6.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure will be described indetail below. Note that the present disclosure is not intended to belimited by the following embodiments. Description in one embodiment andits variant example can be applied to other embodiments and variantexamples unless otherwise specified.

OUTLINE OF EMBODIMENTS

Because ternary quantum dots produced in the common production methodhave not yet been confirmed to exhibit the band-edge emission, theinventors have studied the configuration of ternary quantum dots, inparticular, Group 11-13-16 quantum dots, from which band-edge emissioncan be obtained. During these studies, the inventors have considered thepossibility of producing the band-edge emission by modifying the stateof the surface of ternary or quaternary quantum dots. In particular, theinventors have examined the modification of the surface state of thequantum dot by covering the surface of each of the quantum dots with anappropriate semiconductor. Consequently, the inventors have found thatband-edge emission can be obtained from ternary or quaternary quantumdots each covered by a semiconductor that consists essentially of aGroup 13 element and a Group 16 element, achieving the embodimentsdescribed herein. CdSe, from which the band-edge emission is confirmed,having the surface covered by CdS or the like for further enhancement ofits band-edge emission has already proposed. Also, ternary quantum dotseach having the surface covered by ZnS, ZnSe, or the like has alreadyproposed. However, it has not been reported that employing ternary orquaternary quantum dots, from which band-edge emission is not or barelyobserved in a photoluminescence spectrum, as a core and covering thesurface of the core with a shell made of a specific semiconductor allowsfor obtaining band-edge emission.

Semiconductor nanoparticles in the present embodiment each includes acore and a shell covering the surface of the core. The core is made of asemiconductor that contains M¹, M², and Z, in which M¹ is at least oneelement selected from the group consisting of Ag, Cu, and Au, M² is atleast one element selected from the group consisting of Al, Ga, In andTl, and Z is at least one element selected from the group consisting ofS, Se, and Te. The shell is made of a semiconductor that consistsessentially of a Group 13 element and a Group 16 element and has alarger bandgap energy than the bandgap energy of the core. The shell isin heterojunction with the core. In the description below, the core,which is ternary or quaternary, and the shell will be illustrated.

Core Ternary Core

The core made of a ternary semiconductor is made of semiconductorcontaining M¹, M², and Z, and has a shape of a particle. Thus, the coreis a semiconductor nanoparticle. M¹ is at least one element selectedfrom the group consisting of Ag, Cu, and Au, preferably Ag or Cu, andparticularly preferably Ag. When M¹ is Ag, the core (corresponding to aprimary semiconductor nanoparticle used in a method of producingsemiconductor nanoparticles to be described below) can be easilysynthesized. Two or more elements may be contained as M¹. The crystalstructure of the core may be at least one selected from the groupconsisting of a tetragonal system, a hexagonal system, and anorthorhombic system.

M² is at least one element selected from the group consisting of Al, Ga,In, and Tl, preferably In or Ga, and particularly preferably In. WithIn, a by-product is not easily generated, and thus In is preferable. M²may contain two or more elements.

Z is at least one element selected from the group consisting of S, Se,and Te, and preferably S. The core in which Z is S has a bandgap widerthan that of a core containing Se or Te as the element Z. Accordingly,the core containing S allows for easily emitting photoluminescence of avisible light region, and thus is preferable. Z may contain two or moreelements.

The combinations of M¹, M², and Z may be appropriately selected.Examples of the combination of M¹, M², and Z (typical notation is:M¹/M²/Z) preferably include Cu/In/S, Ag/In/S, Ag/In/Se, and Ag/Ga/S.

A semiconductor containing the above specific elements and having thecrystal structure of the tetragonal system, hexagonal system, ororthorhombic system is generally represented by a composition formula ofM¹M²Z₂, as indicated in various literature and the like. Of thesemiconductors represented by the composition formula of M¹M²Z₂, asemiconductor having the hexagonal system is of the wurtzite type, and asemiconductor having the tetragonal system is of the chalcopyrite type.The crystal structure is identified, for example, by measuring a X-raydiffraction (XRD) pattern obtained by X-ray diffraction. Morespecifically, an XRD pattern obtained from the core is compared with aknown XRD pattern of semiconductor nanoparticles represented by thecomposition formula of M¹M²Z₂, or an XRD pattern determined bysimulation from crystal structure parameters. If one of the knownpatterns or simulated patterns is identical to the pattern of the core,the semiconductor nanoparticle of such a core can be regarded to havethe same crystal structure as that of the identical known or simulatedpattern. The aggregate of the semiconductor nanoparticles may contain aplurality of kinds of nanoparticles with different crystal structures ofthe cores. In that case, the XRD pattern having peaks derived from aplurality of crystal structures is observed.

The core made of a ternary semiconductor does not have a stoichiometriccomposition represented by the general formula above in practice, and inparticular, the ratio of the number of atoms of M¹ to that of M² (i.e.,M¹/M²) is smaller than 1. The sum of the number of atoms of M¹ and thenumber of atoms of M² may not be equal to the number of atoms of Z. Inthe semiconductor nanoparticles of this embodiment, the core made of theternary semiconductor may be made of a semiconductor with such anon-stoichiometric composition. In the present specification, when itdoes not matter whether semiconductors containing specific elements hasa stoichiometric composition or not, the semiconductor composition isrepresented by a formula in which constituent elements are connected bya “—” mark, such as M¹-M²-Z.

The core may be substantially made of only M¹, M², and Z. Note that theterm “substantially” as used herein is used in view of possible presenceof one or greater elements other than the elements M¹, M², and Z thatmay be unintentionally mixed in as impurities or the like.

Alternatively, the core may contain other elements. For example, a partof M² may be substituted with other metal elements. Such other metalelements may be those which form +trivalent metal ions, specifically,one or more elements selected from Cr, Fe, Al, Y, Sc, La, V, Mn, Co, Ni,Ga, In, Rh, Ru, Mo, Nb, W, Bi, As, and Sb. The substitution amount ofthe substitution element is preferably 10% or less when the total numberof atoms of M² and the substitution element is set to 100%.

Quaternary Core

The core made of a quaternary semiconductor is made of a semiconductorcontaining M¹, M², M³, and Z, and is a semiconductor nanoparticle,similar to the core made of a ternary semiconductor. The crystalstructure of the core formed of quaternary semiconductor may be at leastone selected from the group consisting of a tetragonal system, ahexagonal system, and an orthorhombic system. The aggregate of thenanoparticles may contain a plurality of nanoparticles with differentcrystal structures of the cores. In that case, the XRD pattern havingpeaks derived from a plurality of crystal structures is observed. M¹,M², and Z have been described above in relation to the ternary core, andthus their description will be omitted. M³ is at least one elementselected from the group consisting of Zn and Cd. M³ is preferably Zn.When M³ is Zn, the semiconductor nanoparticles in the present embodimentwith a less toxic composition can be provided. Also, when Cd isemployed, the amount of Cd in such a core can be reduced compared to thebinary core using Cd. Thus, the semiconductor nanoparticles of thisembodiment with a less toxic composition can be provided.

The combinations of M¹, M², M³, and Z (typical notation is: M¹/M²/M³/Z)can be appropriately selected. Preferable examples of the combination ofM¹/M²/M³/Z include Cu/In/Zn/S, and Ag/In/Zn/S.

The semiconductor containing the above specific four kinds of elementsand having the crystal structure of the tetragonal system, hexagonalsystem, or orthorhombic system is generally represented by a compositionformula of (M¹M²)_(x)M³ _(y)Z₂ (where x+y=2), as indicated in variousliterature and the like. That is, the semiconductor represented by thiscomposition formula can be regarded as a semiconductor represented bythe composition formula of M¹M²Z₂ described in relation to the core madeof the ternary semiconductor in which M³ is doped or in which M¹M²Z₂ andM³Z form a solid solution.

The core made of the quaternary semiconductor may actually not have astoichiometric composition represented by the general formula above inpractice, and particularly, the ratio of the number of atoms M¹ to thatof M² (i.e., M¹/M²) is smaller than 1. Furthermore, the sum of x and y,namely, x+y may not be 2. In the semiconductor nanoparticles of thepresent embodiment, the core formed of the quaternary semiconductor maybe made of a semiconductor with such a non-stoichiometric composition.In the present specification, when it does not matter whether thequaternary semiconductor has the stoichiometric composition or not, inthe present specification, the semiconductor composition may berepresented by a formula in which constituent elements are connected bya “—” mark, such as M¹-M²-M³-Z. A method of identifying the crystalstructure of the quaternary semiconductor is the same as that explainedin relation to the core made of a ternary semiconductor.

The core made of the quaternary semiconductor may be substantially madeof only M¹, M², M³, and Z. Note that the term “substantially” as usedherein is used in view of possible presence of one or greater elementsother than the elements M¹, M², M³, and Z that may be unintentionallymixed in as impurities or the like. Alternatively, the core made of aquaternary semiconductor may contain other elements.

For example, a part of M² may be substituted with other metal elements.Examples of such other metal elements and the substitution amount orrate thereof have been described above in relation to the ternary core,and thus their description will be omitted.

Additionally or alternatively, a part of M³ may be substituted withother metal elements. Such other metal elements may be those which form+divalent metal ions. More specifically, such other elements may be atleast one element selected from Co, Ni, Pd, Sr, Ba, Fe, Cr, Mn, Cu, Cd,Rh, W, Ru, Pb, Sn, Mg, and Ca. The substitution amount is preferably 10%or less when the total number of atoms of M³ and the substitutionelement is set to 100%.

Shell

In this embodiment, the shell is formed of a semiconductor that has abandgap energy larger than a bandgap energy of the semiconductor formingthe core and that consists essentially of a Group 13 element and a Group16 element. Examples of the Group 13 element can include B, Al, Ga, In,and Tl, and examples of the Group 16 element can include O, S, Se, Te,and Po. The semiconductor constituting the shell may contain one kind ofthe Group 13 element, or two or more kinds of the Group 13 elements, andmay contain one kind of the Group 16 element, or two or more kinds ofthe Group 16 elements. Specifically, the expression “essentially” refersto that the ratio of elements other than the Group 13 element and theGroup 16 element is 5% or less when the total number of atoms of all theelements contained in the shell is set to 100%.

The semiconductor forming the core has a bandgap energy as below. TheGroup 11-13-16 ternary semiconductor or the quaternary semiconductorbased on this ternary semiconductor generally has a bandgap energy of1.0 eV to 3.5 eV. In particular, a semiconductor made of Ag—In—S has abandgap energy of 1.8 eV to 1.9 eV. Furthermore, a semiconductor made ofAg—In—Zn—S has a bandgap energy of 1.8 eV to 3.9 eV. Composition of theshell may be selected in accordance with the bandgap energy of thesemiconductor forming the core. Alternatively, in the case where thecomposition or the like of the shell is determined, the core may bedesigned such that the bandgap energy of the semiconductor forming thecore is smaller than that of the shell.

More specifically, the shell may have a bandgap energy, for example, of2.0 eV to 5.0 eV, and in particular, 2.5 eV to 5.0 eV. The bandgapenergy of the shell may be greater than that of the core, for example,by about 0.1 eV to about 3.0 eV, particularly about 0.3 eV to about 3.0eV, and more particularly about 0.5 eV to about 1.0 eV. With a smalldifference in bandgap energy between the shell and the core, the ratioof photoluminescence emission other than the band-edge emission withrespect to photoluminescence from the core may be increased, leading toa reduction in the ratio of the band-edge emission.

Furthermore, the bandgap energies of the core and the shell arepreferably selected in such a manner as to form a type-I band alignment,in which the bandgap energy of the core is between the bandgap energy ofthe shell in the heterojunction between the core and the shell. With thetype-I band alignment, the band-edge emission from the core can be moreeffectively obtained. In the type-I band alignment, between the bandgapof the core and the bandgap of the shell, a barrier of preferably atleast 0.1 eV, particularly 0.2 eV or higher, and more particularly 0.3eV or higher may be generated. The upper limit of the barrier is, forexample, 1.8 eV, particularly, 1.1 eV. If the barrier is small, theratio of photoluminescence other than the band-edge emission withrespect to the photoluminescence emission from the core may beincreased, which may lead to a reduction in the ratio of the band-edgeemission.

In the present embodiment, the shell may contain In or Ga as the Group13 element. In this embodiment, the shell may contain S as the Group 16element. A semiconductor containing In or Ga, or containing S tends tohave a bandgap energy larger than that of the Group 11-13-16 ternarysemiconductor.

The semiconductor forming the shell may have a crystal system thatmatches the crystal system of the core semiconductor, and may havelattice constant identical to or close to that of semiconductor formingthe core. The shell made of the semiconductor having the crystal systemthat matches that of the core and having the lattice constant close tothat of the core may efficiently cover the periphery of the core. Here,the shell in which a multiple of the lattice constant of the shell isclose to the lattice constant of the core is also referred to as “theshell with the lattice constant close to that of the semiconductorforming the core”. For example, Ag—In—S, which is a Group 11-13-16ternary semiconductor, generally has a tetragonal system. Examples of acrystal system that matches a tetragonal system include a tetragonalsystem and an orthorhombic system. When the Ag—In—S semiconductor has atetragonal system, its lattice constants are 5.828 Å, 5.828 Å, and 11.19Å. It is preferable that the shell covering this semiconductor has atetragonal system or a cubic system whose lattice constant or a multipleof the lattice constant is close to the lattice constant of Ag—In—Ssemiconductor. Alternatively, the shell may be amorphous.

Whether the amorphous shell is formed or not can be confirmed byobserving the core-shell semiconductor nanoparticles of this embodimentwith a high-angle annular dark field (HAADF)-scanning transmissionelectron microscope (STEM). More specifically, in an HAADF-STEM image, aportion with a regular pattern (e.g., a striped pattern or a dotpattern) is observed at the center, while a portion with no regularpattern is observed at the periphery of the portion with the regularpattern. With the HAADFE-STEM, a regular structure, such as crystalmaterial, is observed as an image with a regular pattern, while anon-regular structure, such as amorphous material, is not observed as animage with a regular pattern. For this reason, when the shell isamorphous, the shell can be observed as a part definitely different fromthe core, which is observed as an image with a regular pattern (as ithas the crystal structure, such as a tetragonal system, as mentionedabove).

When the core is made of Ag—In—S, and the shell is made of GaS, in theimage obtained by the HAADF-STEM, the shell tends to be observed as animage part darker than that of the core because Ga is an element with aweight smaller than each of Ag and In.

Whether the amorphous shell is formed or not can also be confirmed byobserving the core-shell semiconductor nanoparticles of the presentembodiment with a high-resolution transmission electron microscope(HRTEM). In an image obtained by the HRTEM, the core part is observed asa crystal lattice image (i.e., image with a regular pattern). Incontrast, the shell part is not observed as a crystal lattice image. Inthe shell part, the contrast between white and black parts can beobserved, but no regular pattern can be observed.

The shell preferably does not form a solid solution with the core. Ifthe shell is solid-soluted with the core, both the core and shell areintegrated into a single body. With this arrangement, a mechanism of thepresent embodiment, in which covering the core with the shell to modifythe surface state of the core allows for exhibiting band-edge emission,is not obtained. For example, it is confirmed that the core made ofAg—In—S covered with zinc sulfide (Zn—S) having a stoichiometriccomposition or a non-stoichiometric composition does not exhibitband-edge emission from the core. Zn—S satisfies the above-describedcondition for the bandgap energy with regard to the relationship withAg—In—S and forms the type-1 band alignment. Nevertheless, no band-edgeemission is obtained from the above-described specific semiconductor. Itis assumed that this is because ZnS and the above-described specificsemiconductor form a solid solution, which eliminates the interfacebetween the core and the shell.

Examples of a combination of a Group 13 element and a Group 16 elementof the shell may include a combination of In and S, a combination of Gaand S, and a combination of In and Ga and S. The combination is notlimited thereto. The combination of In and S may be indium sulfide. Thecombination of Ga and S may be gallium sulfide. The combination of In,Ga and S may be indium gallium sulfide. In the present embodiment,indium sulfide forming the shell may not be stoichiometric (that is,In₂S₃), and thus may be represented by the formula of InS_(x) (where xis any number that is not limited to an integer number, e.g., in a rangeof 0.8 to 1.5) in the present specification. Likewise, gallium sulfidemay not be stoichiometric (that is, Ga₂S₃), and thus may be representedby the formula of GaS_(x) (where x is any number that is not limited toan integer number, e.g., 0.8 to 1.5) in the present specification.Indium gallium sulfide may have a composition represented byIn_(2(1-y))Ga_(2y)S₃ (where y is any number that is greater than 0 andless than 1) or a composition represented by In_(a)Ga_(1-a)S_(b) (wherea is an any number that is greater than 0 and less than 1, and b is anynumber that is not limited to integer number).

Indium sulfide has a bandgap energy of 2.0 eV to 2.4 eV. Indium sulfidewith a cubic crystal system has a lattice constant of 10.775 Å. Galliumsulfide has a bandgap energy of 2.5 eV to 2.6 eV. Gallium sulfide with atetragonal crystal system has a lattice constant is 5.215 Å. All valuesabout the crystal systems and the like described above are reportedvalues, and thus values of the shell in actual core-shell semiconductornanoparticles may not correspond with these reported values. In the casewhere the core is made of Group 11-13-16 ternary or quaternarysemiconductors, in particular, Ag—In—S or Ag—In—Zn—S, indium sulfide andgallium sulfide are preferably used for a semiconductor forming theshell. In particular, gallium sulfide is preferably used because of itslarge bandgap energy. With use of gallium sulfide, band-edge emissionstronger than that in the case of using indium sulfide can be obtained.

The surface of the shell may be modified by any appropriate compound. Inthe case where the surface of the shell is an exposed surface of thecore-shell semiconductor nanoparticle, modification of the exposedsurface allows for stabilizing the nanoparticle, and preventing theagglomeration and growth of the semiconductor nanoparticles, and/orimproving the dispersibility of the semiconductor nanoparticles in thesolvent.

Examples of a surface modifier may include a nitrogen-containingcompound having a hydrocarbon group with a carbon number of 4 to 20, asulfur-containing compound having a hydrocarbon group with a carbonnumber of 4 to 20, and an oxygen-containing compound having ahydrocarbon group with a carbon number of 4 to 20. Examples of thehydrocarbon group with the carbon number of 4 to 20 can includesaturated aliphatic hydrocarbon groups, such as an n-butyl group, anisobutyl group, an n-pentyl group, an n-hexyl group, an octyl group, adecyl group, a dodecyl group, a hexadecyl group, and an octadecyl group;unsaturated aliphatic hydrocarbon groups, such as an oleyl group;alicyclic hydrocarbon groups, such as a cyclopentyl group and acyclohexyl group; and aromatic hydrocarbon groups, such as a phenylgroup, a benzyl group, a naphthyl group, and a naphthylmethyl group.Among them, the saturated aliphatic hydrocarbon groups and theunsaturated aliphatic hydrocarbon groups are preferable. Examples ofnitrogen-containing compounds include amines and amides. Examples ofsulfur-containing compounds include thiols. Examples ofoxygen-containing compounds include fatty acids and the like.

The surface modifier is preferably a nitrogen-containing compound havingthe hydrocarbon group with the carbon number of 4 to 20. Examples ofsuch a nitrogen-containing compound include alkyl amines, such asn-butyl amine, isobutyl amine, n-pentyl amine, n-hexyl amine, octylamine, decyl amine, dodecyl amine, hexadecyl amine, and octadecyl amine,and alkenyl amines, such as oleyl amine. In particular, in view of easyavailability of that of high purity and boiling point of greater than290° C., n-tetradecylamine is preferably employed.

For the surface modifier, a sulfur containing compound having thehydrocarbon group with the carbon number of 4 to 20 is preferably used.Examples of such a sulfur containing compound include n-butanethiol,isobutanethiol, n-pentanethiol, n-hexanethiol, octanethiol, decanethiol,dodecanethiol, hexadecanethiol, and octadecanethiol.

For the surface modifier, a combination of two or more different surfacemodifiers may be used. For example, one compound (e.g., oleylamine)selected from the nitrogen-containing compounds exemplified above, andone compound (e.g., dodecanethiol) selected from the sulfur-containingcompounds exemplified above may be used in combination.

Core-Shell Structure

In the description below, the structure of a core-shell semiconductornanoparticle according to this embodiment, which is configured of thecore and the shell as described above will be illustrated.

The core-shell semiconductor nanoparticles in the present embodiment mayhave an average particle size of 50 nm or less. The average particlesize may be in a range of 1 nm to 20 nm, and particularly in a range of1 nm to 10 nm.

The average particle size of the nanoparticles may be determined, forexample, from a TEM image taken with a transmission electron microscope(TEM). More specifically, the particle size of a nanoparticle refers to,in the TEM image, the longest one of line segments that connects any twopoints at the outer periphery of a single particle and passes throughthe center of the particle.

In the case where the particle has a rod shape, the particle sizethereof refers to the length of the short axis. In the presentspecification, the expression “rod-shaped” refers to a shape that isobserved to have a quadrangular shape such as a rectangular shape (witha cross section of a circular shape, an elliptical shape, or a polygonalshape), an elliptical shape, or a polygonal shape (for example, apencil-like shape), and have a ratio of the long axis to the short axisof more than 1.2. The “long axis” of a rod-shaped particle having anelliptical shape refers to the length of the longest one of linesegments each connecting any two points on the periphery of theparticle. Also, the “long axis” of a rod-shaped particle having aquadrangular or polygonal shape refers to the length of the longest oneof line segments that are in parallel with the longest side of the sidesdefining the periphery of the particle and connect two points on theperiphery of the particle. Meanwhile, the “short axis” indicates thelength of the longest one of line segments that are perpendicular to theline segment defining the long axis and connect two points on theperiphery of the particle.

The average particle size is an arithmetic average of the particle sizesdetermined by taking TEM images of the nanoparticles at a magnificationof 50,000× to 150,000× at three or more sites on a grid appropriatelyselected, and measuring the particle sizes of all the measurablenanoparticles observed in the obtained TEM images. Here, the “measurableparticle” refers to a particle which can be observed as a whole in theTEM image. Thus, a particle that is partially “cut” or not included inan image range is not regarded as a “measureable particle”. The numberof TEM images is selected to be three or more, and the total of thenumber of nanoparticles included in the selected TEM images is selectedto be 100 or greater. Thus, in the case where 100 or more nanoparticlesare shown in three TEM images but actually less than 100 nanoparticlesare present, it is necessary to increase the number of sites to becaptured. Alternatively, even though 100 or more nanoparticles in totalare shown in two TEM images, three or more TEM images is used to measurethe average particle size of the nanoparticles.

In the core-shell semiconductor nanoparticle, the core may have anaverage particle size of, for example, 10 nm or less, and particularly,8 nm or less. The average particle size of the core may be in a range of2 nm to 10 nm, particularly 2 nm to 8 nm, more particularly 2 nm to 6nm, or alternatively in a range of 4 nm to 10 nm or in a range of 5 nmto 8 nm. If the average particle size of the core is too large, thequantum size effect is not easily exhibited, so that band-edge emissionis not easily obtained.

The shell may have a thickness of 0.1 nm or more. The thickness of theshell may be in a range of 0.1 nm to 50 nm, particularly 0.1 nm to 20nm, and more particularly 0.2 nm to 10 nm. Alternatively, the thicknessof the shell may be in a range of 0.1 nm to 3 nm, and particularly 0.3nm to 3 nm. If the thickness of the shell is too small, the effectobtained by coating the core with the shell may not be sufficientlyexhibited, so that band-edge emission is not easily obtained.

The average particle size of the core and the thickness of the shell maybe determined by observing the core-shell semiconductor nanoparticle,for example, with the HAADF-STEM. In particular, in the case where theshell is amorphous, the shell is easily observed as a different partfrom the core, so that the thickness of the shell can be determinedeasily using the HAADF-STEM. In that case, the particle size of the corecan be determined by the method described above for that of thesemiconductor nanoparticle. In the case where the thickness of the shellis not uniform, the thickness of the shell in the particle refers to thesmallest thickness among the measured values.

Alternatively, the average particle size of the core may be measured inadvance before being coated with the shell. Then, an average particlesize of the core-shell semiconductor nanoparticles may be measured.Subsequently, a difference between the average particle size of thecore-shell semiconductor nanoparticles and the average particle size ofthe core measured in advance may be calculated to determine thethickness of the shell.

The semiconductor nanoparticles of the present embodiment emitphotoluminescence upon being irradiated with light. More specifically,when irradiated with ultraviolet light, visible light, or infraredlight, the semiconductor nanoparticles emit light with a wavelengthlonger than that of the irradiated light. The semiconductornanoparticles in the present embodiment may emit fluorescence or lightcontaining fluorescence. In that case, the semiconductor nanoparticlesin the present embodiment can serve as a phosphor for emittingfluorescence. In the semiconductor nanoparticles according to thepresent embodiment, the core is coated with the shell containingelements of specific Groups, allowing for producing band-edge emissionfrom the core that would otherwise fail to generate band-edge emissionif the core is not covered with the shell. More specifically, upon beingirradiated with ultraviolet light, visible light, or infrared light, thesemiconductor nanoparticles in the present embodiment can emitphotoluminescence having a wavelength longer than that of the irradiatedlight, having a main element whose photoluminescence lifetime is 200 nsor less, and/or having a full width at half maximum of anphotoluminescence spectrum of 50 nm or less. The term “photoluminescencelifetime” as used herein refers to a lifetime of photoluminescencemeasured by a device, called fluorescence lifetime measurement device,as will be described in Examples below.

The value of “photoluminescence lifetime of the main component” isdetermined in the procedure described below. First, semiconductornanoparticles are irradiated with an excitation light to emitphotoluminescence. Observing light with wavelengths around the peak ofthe emission spectrum, for example, in a range of (peak wavelength±50nm), a change in the decay of the light (afterglow) is measured overtime. The measurement of the change over time starts from a timing whenthe irradiation with the excitation light is stopped. In general, adecay curve is the sum of a plurality of decay curves derived fromrelaxation processes of such as photoluminescence emission or heat.Thus, in the present embodiment, on the assumption that the obtaineddecay curve contains three components (i.e., three decay curves),parameter fitting is performed such that the three-component decay curveis represented by the following formula where I(t) represents intensityof photoluminescence. The parameter fitting is performed using adedicated software.

I(t)=A ₁exp(−t/τ ₁)+A ₂exp(−t/τ ₂)+A ₃exp(−t/τ ₃)

In the above formula, each of τ₁, τ₂, and τ₃ of the componentsrepresents the time required for attenuation of the light intensity to1/e (36.8%) of the corresponding initial level, which corresponds to thephotoluminescence lifetime of each component. The times τ₁, τ₂, and τ₃are named in the increasing order of the photoluminescence lifetime. A₁,A₂, and A₃ represent contribution rates of the respective components. Inthe present embodiment, when the component having the maximum integralof the curve represented by A_(x) exp(−t/τ_(x)) is assumed as the maincomponent, the photoluminescence lifetime τ of the main component is 200ns or less. Such photoluminescence is assumed to be the band-edgeemission. When identifying the main component, the values A_(x)×τ_(x) ofthe components, which are obtained by integrating A_(x) exp(−t/τ_(x))from 0 to infinity with respect to t, are compared to one another, andthen the component with the largest A_(x)×τ_(x) is determined as themain component.

In this embodiment, the decay curves obtained by performing theparameter fitting assuming that the decay curve contains three, four, orfive components, respectively, do not greatly differ from each other interms of the deviation from an actual decay curve. For this reason, inthe present embodiment, the number of components included in the decaycurve of photoluminescence is assumed to be three when thephotoluminescence lifetime of the main component is determined, allowingfor avoiding the complexity of the parameter fitting.

The band-edge emission may be obtained together with defectluminescence. The defect luminescence generally has a longphotoluminescence lifetime and shows a broad spectrum whose peak ispositioned on the longer-wavelength side with respect to the band-edgeemission. Alternatively, the semiconductor nanoparticles in the presentembodiment may emit only the defect luminescence without any band-edgeemission. Even with such semiconductor nanoparticles, the shell coveringthe core allows for removing an electric defect on the surface of thecore. Thus, the semiconductor nanoparticles can be applied as a materialsuitably used to conduct charge separation and to suppress recombinationfor photoelectric conversion (in particular, a constituent material of asolar battery).

Intensity of the band-edge emission and the position of the peak of theband-edge emission from the semiconductor nanoparticles in the presentembodiment can be changed by adjusting the particle size of thesemiconductor nanoparticle. For example, by decreasing the particle sizeof the semiconductor nanoparticle, the intensity of the band-edgeemission (ratio of intensity of the band-edge emission/intensity ofdefect luminescence) can be further increased. When the particle size ofthe semiconductor nanoparticle is further decreased, the peak wavelengthof the band-edge emission tends to be shifted toward theshort-wavelength side. Furthermore, when the particle size of thesemiconductor nanoparticle is further decreased, a full width at halfmaximum of the spectrum of the band-edge emission tends to be decreased.

The semiconductor nanoparticles of the present embodiment preferably hasabsorption spectrum or excitation spectrum (also referred to as a“fluorescence excitation spectrum”) in which an exciton peak is present.The exciton peak is a peak obtained by generation of an exciton. Thepresence of this peak in the absorption spectrum or excitation spectrumindicates that the semiconductor nanoparticles have the smalldistribution of particle sizes and has less crystal defects, and thusthey are suitable for the band-edge emission. The sharper the excitonpeak is, the greater amount of particles having uniform particle sizeand less crystal defects the aggregate of semiconductor nanoparticlescontains. This is assumed to narrow the full width at half maximum ofthe photoluminescence from the nanoparticles, thereby improving theluminous efficiency. In the absorption spectrum or excitation spectrumof the semiconductor nanoparticles in this embodiment, the exciton peakis observed, for example, in region between 350 nm and 1,000 nm. Theexcitation spectrum for observing the presence of the exciton peak maybe measured at an observation wavelength in the vicinity of the peakwavelength.

Method of Producing Core-Shell Semiconductor Nanoparticles

The method of producing the core-shell semiconductor nanoparticles inthis embodiment includes:

providing a dispersion by dispersing semiconductor nanoparticles each ofwhich is to be the core (hereinafter the core before being coated with ashell is referred to as “primary semiconductor nanoparticles” forconvenience) into a solvent; and

adding, to the dispersion, a compound containing a Group 13 element as asource thereof and an elemental substance of a Group 16 element or acompound containing a Group 16 element as a source thereof to form asemiconductor layer consisting essentially of the Group 13 element andthe Group 16 element on the surface of each of the primary semiconductornanoparticles.

Preparation of Primary Semiconductor Nanoparticles

The primary semiconductor nanoparticle corresponds to the core asdescribed above. When the core is a ternary semiconductor, the primarysemiconductor nanoparticle is a nanoparticle made of a semiconductorthat contains M¹, M², and Z. When the core is a quaternarysemiconductor, the primary semiconductor nanoparticle is a nanoparticlemade of a semiconductor that contains M¹, M², M³, and Z. For the primarysemiconductor nanoparticles, a commercially available product may beused, or one produced on a trial basis may be used. Alternatively, theprimary semiconductor nanoparticles may be produced by reacting an M¹source, an M² source, and a Z source, and optionally, an M³ source. Inthe production method according to the present embodiment, coating withthe shell may not necessarily be performed immediately after producingor providing the primary semiconductor nanoparticles. Primarysemiconductor nanoparticles that are separately produced and left for awhile may be used.

For example, the ternary primary semiconductor nanoparticles may beproduced by a method in which a salt of the element M¹, a salt of theelement M², and a ligand containing the element Z as a coordinationelement are mixed to give a complex, and then the complex isheat-treated. Any kinds of a salt of M¹ and a salt of M² may beemployed, and either an organic acid salt or an inorganic acid salt maybe employed. More specifically, the salt may be any one of a nitratesalt, an acetate salt, a hydrosulfate salt, a hydrochloride salt, and asulfonate salt, and is preferably an organic acid salt, such as anacetate salt. This is because the organic acid salt has high solubilityin an organic solvent, which allows the reaction to proceed uniformly.

When Z is sulfur (S), examples of the ligand containing the element Z asthe coordination element include ß-dithiones, such as2,4-pentanedithione; dithiols, such as1,2-bis(trifluoromethyl)ethylene-1,2-dithiol; diethyldithiocarbamate;and thiourea.

When Z is tellurium (Te), examples of the ligand containing the elementZ as the coordination element include diallyl telluride, and dimethylditelluride. When Z is selenium (Se), examples of the ligand containingthe element Z as the coordination element includedimethyldiselenocarbamic acid, and 2-(dimethylamino)ethaneselenol.

The complex is obtained by mixing the salt of M¹, the salt of M², andthe ligand containing the element Z as the coordination element. Thecomplex may be formed by a method in which an aqueous solutioncontaining the salt of M¹ and the salt of M² is mixed with an aqueoussolution of the ligand, or alternatively by a method in which the saltof M¹, the salt of M², and the ligand are introduced into an organicsolvent (in particular, an organic solvent with high polarity, such asethanol) and then mixed. The organic solvent may be the surface modifieror a solution containing the surface modifier. The charging ratio of thesalt of M¹, the salt of M², and the ligand containing the element Z asthe coordination element is preferably set at 1:1:2 (in terms of molarratio) corresponding to the composition of M¹M²Z₂.

Next, the obtained complex is subjected to heat treatment to form theprimary semiconductor nanoparticles. More specifically, the heattreatment of the complex may be performed by precipitating the obtainedcomplex to separate the complex from the solvent, followed by drying itinto powder, and heating the powder, for example, at a temperature in arange of 100° C. to 300° C. In this case, preferably, the obtainedprimary semiconductor nanoparticles are subjected to another heattreatment in a solvent, which is the surface modifier, or a solventcontaining a surface modifier, so that the surfaces of the nanoparticlesare modified. Alternatively, the heat treatment of the complex may beperformed by heating the complex obtained in the form of powder in asolvent which is the surface modifier or a solvent containing thesurface modifier, for example, at a temperature in a range of 100° C. to300° C. Alternatively, in the case where the complex is formed byintroducing the salt of M¹, salt of M², and the ligand into an organicsolvent and mixing these, the salts and the ligand may be introducedinto the organic solvent, which is a surface modifier or a solventcontaining the surface modifier, and then subjected to the heattreatment, which allows for continuously or simultaneously performingthe complex formation, the heat treatment, and the surface modification.

Alternatively, the primary semiconductor nanoparticles may be formed byintroducing the salt of M¹, the salt of M², and a source compound of Zinto an organic solvent. Alternatively, the primary semiconductornanoparticles may be produced by a method that includes forming acomplex by a reaction between the organic solvent and the salt of M¹,forming another complex by a reaction between the organic solvent andthe salt of M², reacting these complexes with the source compound of Zto produce a reaction product, and growing crystals of the obtainedreaction product. The salt of M¹ and the salt of M² have been describedabove in relation to the production method including the formation ofcomplex. Examples of the organic solvent for forming the complex byreaction with these salts include alkylamines, alkenylamines,alkylthiols, alkenylamine, alkylphosphines, and alkenylphosphines havingthe carbon number of 4 to 20. These organic solvents finally serve tomodify the surfaces of the obtained primary semiconductor nanoparticles.Such organic solvents may be used mixed with other organic solvents.Also in this production method, the charging ratio of the salt of M¹,the salt of M², and the source compound of Z is preferably 1:1:2 (interms of molar ratio) corresponding to the composition of M¹M²Z₂.

When Z is sulfur (S), examples of a source compound of Z include sulfur,thiourea, thioacetamide, and alkylthiol. When Z is tellurium (Te), forexample, a suspension in which Te powder is added into atrialkylphosphine is subjected to heat treatment at a temperature in arange of 200° C. to 250° C. to produce a Te-phosphine complex, which maybe used as the source compound of Z. When Z is selenium (Se), forexample, a suspension in which Se powder is added into atrialkylphosphine is subjected to heat treatment at a temperature of200° C. to 250° C. to produce a Se-phosphine complex, which may be usedas the source compound of Z.

Alternatively, primary semiconductor nanoparticles may be produced byusing a so-called hot-injection method. The hot-injection method is amethod of producing semiconductor nanoparticles in which a solution(also called as a precursor solution) into which source compounds ofrespective elements for forming primary semiconductor nanoparticles(e.g., a salt of M¹, a salt of M², and a source compound of Z or aligand containing the element Z as the coordination element) aredissolved or dispersed is charged into a solvent heated at a temperaturein a range of 100° C. to 300° C. for a short time (e.g. millisecondsorder), thereby forming a number of crystal nuclei in an initialreaction stage. Alternatively, the hot-injection method may involve:dissolving or dispersing source compounds of some of the elements in anorganic solvent, followed by heating it; and subsequently charging aprecursor solution of the remaining elements into the organic solvent.In the case where the solvent is the surface modifier, or a solventcontaining the surface modifier, the modification of the surfaces of theparticles can be performed simultaneously. With such a hot-injectionmethod, the nanoparticles each with a smaller particle size can bemanufactured.

The surface modifier for modifying the surface of each of the primarysemiconductor nanoparticles has been described above in relation to theshell. With the primary semiconductor nanoparticles each having themodified surface, the particles are stabilized, which can prevent theagglomeration or growth of the particles, and/or can improve thedispersibility of the particles in the solvent. In the case where thesurfaces of the primary semiconductor nanoparticles are modified, thegrowth of the shell starts when the surface modifier is removed. Thus,generally, the surface modifier that modifies the primary semiconductornanoparticles is not present on the surface of the core (at theinterface between the core and shell) in the core-shell nanoparticlesfinally obtained.

In the case of employing any of these methods, the primary semiconductornanoparticles are produced under an inert atmosphere, particularly,under an argon atmosphere or a nitrogen atmosphere. This is for reducingor preventing the subgeneration of oxides and the oxidation of thesurface of the primary semiconductor nanoparticle.

In the case where the primary semiconductor nanoparticles are formed tofurther contain the element M³, in addition to the elements M¹ and M²and the element Z, i.e., when the primary semiconductor nanoparticlesare made of the quaternary semiconductor, a salt of M³ is used togetherwith the salt of M¹, and the salt of M² in the production of the primarysemiconductor nanoparticles mentioned above. The charging ratio of thesalt of M¹, the salt of M², the salt of M³, and the ligand containingthe element Z as the coordination element or the source compound of Z ispreferably x:x:y:2 (in terms of molar ratio) that corresponds to acomposition formula of the stoichiometric composition, namely,(M¹M²)_(x)M³ _(y)Te₂ (where x+y=2). The salt of M³ may be either anorganic acid salt or an inorganic acid salt. More specifically, the saltof M³ may be any one of a nitrate salt, an acetate salt, a hydrosulfatesalt, a hydrochloride salt, and a sulfonate salt. Other configurationsregarding the method of producing the quaternary primary semiconductornanoparticles are substantially the same as that for the ternary primarysemiconductor nanoparticles, and thus its detailed description will beomitted.

In the case where a part of M² of the ternary or quaternarysemiconductor nanoparticles is substituted with another metal element, asalt of another metal element is used at the time of producing theprimary semiconductor nanoparticles. In this case, the charging ratio ofthe salt of M² and the salt of the metal element is adjusted such thatthe substitution amount of the metal element is at a desired value.Likewise, in the case of producing the quaternary semiconductornanoparticles having a composition in which a part of M³ is substitutedwith another metal element, the salt of another metal element is used inproducing of the primary semiconductor nanoparticles.

The ternary or quaternary semiconductor nanoparticles produced in theabove-described manner may be separated from the solution after the endof the reaction, and may be refined as needed. The separation isperformed, for example, by centrifuging a suspension after producing theparticles, and taking out a supernatant. The refinement is performed byadding alcohol to the supernatant, centrifuging the supernatant togenerate precipitates, taking out the precipitates (or removing thesupernatant), and then drying the separated precipitates, for example,by vacuum deaeration or natural drying, or dissolving the precipitatesin an organic solvent. The refinement (including the addition of alcoholand the centrifugation) may be performed a plurality of times as needed.Examples of an alcohol for use in the refinement include a loweralcohol, such as methanol, ethanol, and n-propanol. In the case ofdissolving the precipitates in an organic solvent, examples of organicsolvents include chloroform, toluene, cyclohexane, hexane, pentane, andoctane.

In the case of drying the separated precipitates after the refinement,the drying may be performed, for example, by vacuum deaeration ornatural drying, or alternatively, by a combination of the vacuumdeaeration and natural drying. The natural drying may be performed, forexample, by leaving the particles in the atmosphere at normaltemperature and pressure. In this case, the particles are left for 20hours or more, for example, for approximately 30 hours.

Dispersion of Primary Semiconductor Nanoparticles

In the covering of the primary semiconductor nanoparticles with theshell, the primary semiconductor nanoparticles are dispersed in anappropriate solvent to prepare a dispersion. Subsequently, in thedispersion, a semiconductor layer is formed as the shell.Light-scattering does not occur in the liquid in which the primarysemiconductor nanoparticles are dispersed. Thus, the obtained dispersionis generally transparent (colored or colorless). Any appropriate solventmay be used for dispersing the primary semiconductor nanoparticlesthereinto. As in the production of the primary semiconductornanoparticles, an appropriate organic solvent (in particular, an organicsolvent with high polarity, such as ethanol) may be used. The organicsolvent may be the surface modifier or a solution containing the surfacemodifier. For example, the organic solvent may be at least one selectedfrom nitrogen-containing compounds having a hydrocarbon group with thecarbon number of 4 to 20; or at least one selected fromsulfur-containing compounds having a hydrocarbon group with the carbonnumber of 4 to 20; or a combination of at least one selected from thenitrogen-containing compounds having a hydrocarbon group with the carbonnumber of 4 to 20 and at least one selected from the sulfur-containingcompounds having a hydrocarbon group with the carbon number of 4 to 20.These exemplified compounds are described as the surface modifier inrelation to the shell. As the nitrogen-containing compound,n-tetradecylamine is preferable in view of easy availability of ahigh-purity one and boiling point of greater than 290° C. Morespecifically, examples of the organic solvent include oleylamine,n-tetradecylamine, dodecanethiol, and combinations thereof.

The dispersion of the primary semiconductor nanoparticles may beadjusted such that the proportion of the nanoparticles in the dispersionis, for example, 0.02% by mass to 1% by mass, and particularly 0.1% bymass to 0.6% by mass. If the proportion of the particles in thedispersion is too low, products are difficult to be collected inaggregation and precipitation processes by a poor solvent. If theproportion of the particles in the dispersion is too high, rate ofOstwald ripening and fusion of particles of the materials forming thecore may be increased, which tends to widen the particle sizedistribution.

Formation of Shell

The formation of the semiconductor layer as the shell is performed byadding a compound containing a Group 13 element and either an elementalsubstance of a Group 16 element or a compound containing a Group 16element to the dispersion described above. The compound containing theGroup 13 element serves as a Group 13 element source, and is, forexample, an organic salt, an inorganic salt, an organometallic compoundof the Group 13, etc. More specifically, examples of the compoundcontaining the Group 13 element include nitrate salts, acetate salts,sulfate salts, hydrochloride salts, sulfonate salts, and acetylacetonatocomplexes, and preferably organic salts, such as acetate salts, ororganometallic compounds. This is because the organic salt andorganometallic compound have high solubility into the organic solvent,which can facilitate uniformly proceeding the reaction.

An elemental substance of the Group 16 element or a compound containingthe Group 16 element serves as a Group 16 element source. For example,in the case where sulfur (S) is a constituent element of the shell asthe Group 16 element, the elemental sulfur, such as high-purity sulfur,can be used, or sulfur-containing compounds can be used. Examples ofsuch a sulfur-containing compound include thiols, such as n-butanethiol,isobutanethiol, n-pentanethiol, n-hexanethiol, octanethiol, decanethiol,dodecanethiol, hexadecanethiol, and octadecanethiol; disulfides such asdib enzyldisulfide; thiourea; and thiocarbonyl compounds.

In the case where oxygen (O) is a constituent element of the shell asthe Group 16 element, alcohols, ethers, carboxylic acids, ketones, orN-oxide compounds may be used as a Group 16 element source. In the casewhere selenium (Se) is a constituent element of the shell as the Group16 element, an elemental selenium, or compounds, such as selenizedphosphine oxides, organic selenium compounds (dibenzyldiselenide,diphenyldiselenide) or selenium hydrides, may be used as a Group 16element source. In the case where tellurium (Te) is a constituentelement of the shell as the Group 16 element, an elemental tellurium,tellurium phosphine oxides, or tellurium hydrides, may be used as a theGroup 16 element source.

Any appropriate method may be employed for adding the Group 13 elementsource and the Group 16 element source into the dispersion. For example,a mixture in which a Group 13 element source and a Group 16 elementsource may be dispersed or dissolved into an organic solvent. Then, themixture may be added to a dispersion little by little, for example, bydropping. In this case, the mixture may be added at a rate of 0.1 mL/hrto 10 mL/hr, and particularly at a rate of 1 mL/hr to 5 mL/hr. Themixture may be added to the heated dispersion. More specifically, forexample, temperature of the dispersion may be increased to a peaktemperature of 200° C. to 290° C. After reaching the peak temperature,the mixture may be added little by little to the dispersion whileholding the peak temperature, followed by decreasing the temperature ofthe dispersion with the mixture added, so that a shell layer may beformed (namely, a slow injection method). The peak temperature may beheld if necessary even after the addition of the mixture is finished.

If the peak temperature is too low, the surface modifier that modifiesthe primary semiconductor nanoparticles cannot be removed sufficiently,or a chemical reaction for forming the shell does not proceedsufficiently. For these reasons, the semiconductor layer (shell) is notformed sufficiently in some cases. On the other hand, if the peaktemperature is extremely high, the primary semiconductor nanoparticlessometimes have their quality altered, whereby even the formation of theshell cannot produce the band-edge emission in some cases. The time forholding the peak temperature may be set from one minute to 300 minutes,and particularly 10 minutes to 60 minutes in total after the start ofthe addition of the mixture. The time for holding at the peaktemperature (i.e., temperature holding time) is selected in view of therelationship with the peak temperature. When the peak temperature islower, the temperature holding time is increased. When the peaktemperature is higher, the temperature holding time is decreased. Inthese manners, good shell layer can be easily formed. The temperatureincreasing rate and the temperature decreasing rate are not particularlylimited. Decreasing of the temperature may be performed, for example, byholding the dispersion in which the mixture is added at the peaktemperature for a predetermined time and then allowing the dispersion tocool by turning off a heating source (e.g., electric heater).

Alternatively, whole amounts of the Group 13 element source and theGroup 16 element source may be added directly to the dispersion. Then,the dispersion to which the Group 13 element source and the Group 16element source are added may be heated, so that the semiconductor layercan be formed as the shell on the surfaces of the primary semiconductornanoparticles (namely, a heating-up method). More specifically, thetemperature of the dispersion into which the Group 13 element source andthe Group 16 element source are added may be gradually increased to apeak temperature of 200° C. to 290° C. After holding at the peaktemperature for one to 300 minutes, the temperature of the dispersionmay be gradually decreased. The temperature increasing rate may be set,for example, at 1° C./min to 50° C./min, and the temperature decreasingrate may be set, for example, to 1° C./min to 100° C./min.Alternatively, heating may be performed to a predetermined peaktemperature without controlling the temperature increasing rate.Alternatively, the temperature decreasing may not be performed at aconstant rate. The temperature decreasing may be performed by turningoff the heating source. The issues in the case of the excessively lowpeak temperature or excessively high peak temperature has been describedin description of the method of adding the mixture.

With the heating-up method, the core-shell semiconductor nanoparticlestend to be obtained, with band-edge emission of an intensity higher thanthat of the core-shell semiconductor nanoparticles in which the shell isformed by the slow-injection method.

The charging ratio of the Group 13 element source and the Group 16element source preferably corresponds to the stoichiometriccompositional ratio of the compound semiconductor consisting of theGroup 13 element and the Group 16 element in any method of adding theGroup 13 element source and the Group 16 element source. For example, inthe case of using an In source as the Group 13 element source and a Ssource as the Group 16 element source, the charging ratio is preferably1:1.5 (In:S) corresponding to a composition formula of In₂S₃. Similarly,in the case of using a Ga source as the Group 13 element source and a Ssource as the Group 16 element source, the charging ratio is preferably1:1.5 (Ga:S) corresponding to the composition formula of Ga₂S₃. However,the charging ratio may not necessarily be the stoichiometriccompositional ratio. In the case where the raw material is charged withan excessive amount compared to the intended produced amount of theshell, the amount of the Group 16 element source may be smaller than thestoichiometric compositional ratio. For example, the charging ratio maybe 1:1 (the Group 13:the Group 16).

The charging amount is selected in view of an amount of the primarysemiconductor nanoparticles contained in a dispersion such that theshell with a desired thickness is formed on the primary semiconductornanoparticles contained in the dispersion. For example, the chargingamounts of the Group 13 element source and the Group 16 element sourcemay be selected such that the 0.01 mmol to 10 mmol, particularly 0.1mmol to 1 mmol of the compound semiconductor consisting of the Group 13element and the Group 16 element with stoichiometric composition issynthesized for the 10 nmol of the primary semiconductor nanoparticles,at amount of particles. The “amount of particles” refers to a molaramount assuming that the single particle is a giant molecule, and isequal to a value obtained by dividing the number of the nanoparticlescontained in the dispersion by Avogadro number (N_(A)=6.022×10²³).

In the production method according to the present embodiment, it ispreferable to form the shell containing indium sulfide or galliumsulfide by using indium acetate or gallium acetylacetonate as the Group13 element source, and using elemental sulfur or dibenzyldisulfide asthe Group 16 element source, and using n-tetradecylamine as thedispersant.

With use of the dibenzyldisulfide for the Group 16 element source(sulfur source) in the heating-up method, the shell is sufficientlyformed and the semiconductor nanoparticles giving intense band-edgeemission are easily obtained, even if the temperature holding time afterreaching the peak temperature is short (for example, 20 minutes to 30minutes). In the case of using the elemental sulfur to form the shell, ashort temperature holding time after reaching the peak temperature maylead to difficulty in obtaining the semiconductor nanoparticles givingintense band-edge emission. However, increasing the temperature holdingtime (for example, 40 minutes or longer, particularly 50 minutes orlonger, and the upper limit being, for example, 60 minutes or shorter)allows for easily obtaining the semiconductor nanoparticles with intenseband-edge emission. Further, in the case where the elemental sulfur isused and the temperature holding time is increased, it is possible toobtain the semiconductor nanoparticles with band-edge emission with aphotoluminescence intensity higher than that of the semiconductornanoparticles produced using dibenzyldisulfide. Furthermore, with theheating-up method in which elemental sulfur is used and the temperatureholding time is increased, it is possible to obtain the semiconductornanoparticles to exhibit photoluminescence spectra having a broad peakintensity due to defect luminescence that is sufficiently smaller thanthe peak intensity of the band-edge emission. Even further, the more thetemperature holding time is increased irrespective of the types ofsulfur sources, the more the peak of the band-edge emission emitted fromthe resultant semiconductor nanoparticles tends to be shifted toward thelonger wavelength side. With use of n-tetradecylamine for thedispersion, it is possible to obtain the semiconductor nanoparticleshaving a broad peak intensity due to defect luminescence that issufficiently smaller than the peak intensity of the band-edge emission.The above tendency is significantly observed in the case where thegallium source is used for the Group 13 element source.

The shell is formed in this manner, so that the core-shell semiconductornanoparticles is formed. The obtained core-shell semiconductornanoparticles may be separated from the solvent. Further, the core-shellsemiconductor nanoparticles may be further refined and dried, if needed.The methods of separation, refinement and drying are as described inconnection with the primary semiconductor nanoparticles, and thus theirdetailed description will be omitted.

Light-Emitting Device

As another embodiment, a light-emitting device will be described inwhich the core-shell semiconductor nanoparticles which have beendescribed above is used. The light-emitting device according to thepresent embodiment is a light-emitting device including a lightconversion member and a semiconductor light-emitting element, whereinthe light conversion member contains the above-described core-shellsemiconductor nanoparticles. In such a light-emitting device, forexample, the core-shell semiconductor nanoparticles absorb a portion oflight emitted from the semiconductor light-emitting element, and thenemit light of a longer wavelength. Light from the core-shellsemiconductor nanoparticles and the rest of light emitted from thesemiconductor light-emitting element are mixed together, and the mixedlight can then be used as the light emission from the light-emittingdevice.

More specifically, with the semiconductor light-emitting elementconfigured to emit blue-violet or blue light with a peak wavelength ofabout 400 nm to about 490 nm and the core-shell semiconductornanoparticles to absorb blue light and emit yellow light therefrom, alight-emitting device configured to emit white light can be obtained.Alternatively, with two kinds of core-shell semiconductor nanoparticles,of which one kind of core-shell semiconductor nanoparticles configuredto absorb blue light and emit green light and the other kind ofcore-shell semiconductor nanoparticles configured to absorb blue lightand emit red light, a white light-emitting device can be obtained.Further, alternatively, with a semiconductor light-emitting elementconfigured to emit ultraviolet light with a peak wavelength of 400 nm orless, and three kinds of core-shell semiconductor nanoparticles thatabsorb the ultraviolet light and emit blue, green, and red lights,respectively, a white light-emitting device can be obtained. In thiscase, to avoid the leak of the ultraviolet light emitted from thelight-emitting element toward the outside, the entirety of light fromthe light-emitting element is desirably absorbed and converted by thesemiconductor nanoparticles.

Moreover, alternatively, with a light-emitting element configured toemit a blue-green light with a peak wavelength of about 490 nm to 510 nmand the core-shell semiconductor nanoparticles to absorb the blue-greenlight and emit red light, a device configured to emit white light can beobtained. Furthermore, alternatively, with a semiconductorlight-emitting element configured to emit red light with a wavelength of700 nm to 780 nm and the core-shell semiconductor nanoparticles toabsorb the red light and emit near-infrared light, a light-emittingdevice configured to emit near-infrared light can be obtained.

The core-shell semiconductor nanoparticles may be used in combinationwith other semiconductor quantum dots, or any phosphors other thanquantum dots (e.g., an organic phosphor or an inorganic phosphor). Othersemiconductor quantum dots may be, for example, binary semiconductorquantum dots mentioned in the Background section in the presentspecification. For the phosphors other than quantum dots, garnet-basedphosphors, such as an aluminum garnet-based phosphor, can be used.Examples of the garnet phosphors include anyttrium-aluminum-garnet-based phosphor activated by cerium, and alutetium-aluminum-garnet-based phosphor activated by cerium. Examples ofthe phosphors other than quantum dots include a nitrogen-containingcalcium aluminosilicate-based phosphor activated by europium and/orchromium; a silicate-based phosphor activated by europium; anitride-based phosphor such as a ß-SiAlON-based phosphor, a CASN-basedphosphor, and a SCASN-based phosphor; a rare earth nitride-basedphosphor such as LnSi₃N₁₁-based phosphor and a LnSiAlON-based phosphor;an oxynitride-based phosphor such as a BaSi₂O₂N₂:Eu-based phosphor andBa₃Si₆O₁₂N₂:Eu-based phosphor; a sulfide-based phosphor such as aCaS-based phosphor, a SrGa₂S₄-based phosphor, a SrAl₂O₄-based phosphor,and a ZnS-based phosphor; a chlorosilicate-based phosphor; SrLiAl₃N₄:Euphosphor; SrMg₃SiN₄:Eu phosphor; and K₂SiF₆:Mn phosphor as a fluoridecomplex phosphor activated by manganese may be used.

The light conversion member which contains the core-shell semiconductornanoparticles in the light-emitting device may have, for example, asheet-like shape or a plate-like shaped member, or may be a member witha three-dimensional shape. Examples of the member with thethree-dimensional shape include a sealing member of a surface-mountedlight-emitting diode in which a semiconductor light-emitting element isarranged at a bottom surface of a recess defined in a package. Thesealing member is formed by charging resin into the recess to seal thelight-emitting element.

Alternatively, other examples of the light conversion member include aresin member formed with a substantially uniform thickness to encloseupper surface and lateral surfaces of a semiconductor light-emittingelement arranged on a planar substrate. Further, alternatively, in thecase where a resin member which contains a reflective material is filledat the periphery of a semiconductor light-emitting element such that anupper end of the resin member is in the same plane with thesemiconductor light-emitting element, further other examples of thelight conversion member include a plate-shaped resin member with apredetermined thickness arranged on an upper side of the semiconductorlight-emitting element and the above-described resin member whichcontains the reflective material.

The light conversion member may be in contact with the semiconductorlight-emitting element, or may be disposed spaced from the semiconductorlight-emitting element. More specifically, the light conversion membermay be a pellet-shaped member, a sheet member, a plate-shaped member, ora stick-shaped member, which is disposed spaced from the semiconductorlight-emitting element. Alternatively, the light conversion member maybe a member which is disposed in contact with the semiconductorlight-emitting element for example, a seal member, a coating member(i.e., member covering the light-emitting element disposed spaced fromthe mold member), or a mold member (e.g., including a lens-shapedmember). In the case of using two or more kinds of core-shellsemiconductor nanoparticles to exhibit photoluminescence with differentwavelengths in the light-emitting device, one light conversion membermay contain a mixture of the two or more kinds of core-shellsemiconductor nanoparticles, or alternatively, a combination of two ormore light conversion members, each of which contains only one kind ofquantum dots, may be used. In this case, two or more kinds of lightconversion members may be layered, or may be arranged on a plane in adot-like or striped pattern.

Examples of the semiconductor light-emitting element include alight-emitting diode (LED) chip. The LED chip may include asemiconductor layer made of one or more kinds of compounds selected fromthe group consisting of GaN, GaAs, InGaN, AlInGaP, GaP, SiC, and ZnO.The semiconductor light-emitting element configured to blue-violetlight, blue light, or ultraviolet light preferably includes, as asemiconductor layer, a GaN based compound represented by a generalformula of In_(X)Al_(Y)Ga_(1-X-Y)N (0≤X, 0≤Y, X+Y<1).

The light-emitting device in this embodiment is preferably incorporatedas a light source in a liquid crystal display device. Since theband-edge emission produced by the core-shell semiconductornanoparticles has a short photoluminescence lifetime, the light-emittingdevice using this core-shell semiconductor nanoparticles is suitable foruse as a light source for the liquid crystal display device thatrequires a relatively high response speed. The core-shell semiconductornanoparticles in the present embodiment can exhibit thephotoluminescence peak with a smaller full width at half maximum as theband-edge emission. Thus, the liquid crystal display device thatexhibits excellent color reproducibility can be obtained without using acolor filter with a dense color, if, in the light-emitting device:

the blue semiconductor light-emitting element is adapted to produce theblue light with a peak wavelength of 420 nm to 490 nm; and thecore-shell semiconductor nanoparticles are adapted to produce the greenphotoluminescence with a peak wavelength of 510 nm to 550 nm, preferably530 nm to 540 nm, and the red photoluminescence with a peak wavelengthof 600 nm to 680 nm, preferably 630 nm to 650 nm, respectively; or

the semiconductor light-emitting element is adapted to produce theultraviolet light with a peak wavelength of 400 nm or less; and thecore-shell semiconductor nanoparticles are adapted to produce the bluephotoluminescence with a peak wavelength of 430 nm to 470 nm, preferably440 nm to 460 nm, the green photoluminescence with a peak wavelength of510 nm to 550 nm, preferably 530 nm to 540 nm, and the redphotoluminescence with a peak wavelength of 600 nm to 680 nm, preferably630 nm to 650 nm, respectively. The light-emitting device of thisembodiment is used, for example, as a direct illumination-type backlightor an edge illumination-type backlight.

Alternatively, a sheet, a plate-shaped member, or a rod-shaped membermade of a resin, glass, or the like, which contains the core-shellsemiconductor nanoparticles, may be incorporated as a light conversionmember independently from the light-emitting device, into a liquidcrystal display device.

EXAMPLES Example 1 (1) Production of Primary Semiconductor Nanoparticles(Ag—In—S)

First, 0.4 mmol of each of silver acetate (AgOAc) and indium acetate(In(OAc)₃), 0.8 mmol of thiourea, 11.8 mL of oleylamine, and 0.2 mL of1-dodecanethiol were charged into a two-necked flask and subjected tovacuum deaeration (for 3 minutes at normal temperature), followed byheating under an Ar atmosphere at a temperature increasing rate of 10°C./min up to 200° C. The obtained suspension was allowed to cool,followed by centrifugal separation (with a radius of 150 mm at 2,500 rpmfor 10 minutes), so that a supernatant dark red liquid was taken out.Then, methanol was added to the supernatant solution until nanoparticleswere precipitated, followed by centrifugal separation (with a radius of150 mm at 2,500 rpm for 3 minutes). Then, the precipitates were dried atthe normal temperature under vacuum, thereby producing semiconductornanoparticles (primary semiconductor nanoparticles).

An XRD pattern of the obtained primary semiconductor nanoparticles wasmeasured and compared with an XRD pattern of AgInS₂ having a tetragonalsystem (of a chalcopyrite type), an XRD pattern of AgInS₂ having ahexagonal system (of a wurtzite type), and an XRD pattern of AGInS₂having an orthorhombic system. The measured XRD pattern is shown inFIG. 1. The XRD pattern shows that the crystal structure of the primarysemiconductor nanoparticles is substantially the same as the tetragonalAgInS₂. The XRD pattern was measured using a powder X-ray diffractometermanufactured by RIGAKU Corporation (trade name: SmartLab). Note that thesame goes for the Examples in the below.

The shapes of the obtained primary semiconductor nanoparticles wereobserved using a transmission electron microscope (TEM, manufactured byHITACH HIGH-TECHNOLOGIES Corporation, trade name: H-7650), and theaverage particle size was measured from TEM images at a magnification of80,000× to 200,000×. Here, the TEM grid available under the trade nameHigh Resolution Carbon HRC-C10 STEM Cu100P grid (manufactured byOKENSHOJI Co., Ltd.) was used. The shape of the obtained particles wasspherical or polygonal. An average particle size was determined byselecting three or more TEM images, then measuring the particle sizes ofall measurable nanoparticles included in these selected images, i.e.,all particles except for nanoparticles whose images were cut at theedges of the images, and calculating an arithmetic average of themeasured particle sizes. In all Examples herein, including Example 1 andComparative Examples, three or more TEM images were used to measure theparticle sizes of 100 or more nanoparticles in total. The averageparticle size of the primary semiconductor nanoparticles was 5.3 nm.

The substance amount of indium contained in the primary semiconductornanoparticles obtained in the above production step (1) was measured byan inductively coupled plasma (ICP) emission spectroscopy (usingICPS-7510 (trade name) manufactured by SHIMAZU Corporation). As aresult, the substance amount of indium was 52.2 μmol. In the case wherethe primary semiconductor nanoparticles each has a spherical shape, thevolume of each of the primary nanoparticles having an average particlesize of 5.3 nm was calculated to be 77.95 nM³. In the case where silverindium sulfide crystal has a tetragonal system, the unit cell volume ofthe silver indium sulfide crystal is calculated to be 0.38 nM³ (latticeconstant: 5.828 Å, 5.828 Å, and 11.19 Å). Accordingly, by dividing thevolume of each of the primary semiconductor nanoparticle by the unitcell volume, one particle of the primary semiconductor nanoparticles wasdetermined to contain 205 unit cells. Next, since one unit cell of thesilver indium sulfide crystal having the tetragonal system contains fourindium atoms, one nanoparticle was calculated to contain 820 indiumatoms. By dividing the substance amount of indium by the number ofindium atoms per nanoparticle, the amount of the primary semiconductornanoparticles was determined to be 63.6 nmol at amount of nanoparticles.

(2) Covering with Shell

Using the primary semiconductor nanoparticles obtained in the step (1),20 nmol of the primary semiconductor nanoparticles at amount ofnanoparticles was dispersed into a mixed solvent that contained 5.9 mLof oleylamine and 0.1 mL of dodecanethiol to obtain a dispersion. Theobtained dispersion was held at 200° C. under an Ar atmosphere.Separately from this, 0.8 mmol (233 mg) of indium acetate (In(OAc)₃) and1.2 mmol (38.4 mg) of sulfur powder were added to 8 mL of oleylamine,and then stirred at 100° C. for 10 minutes, so that the indium acetateand sulfur powder were dissolved to produce a mixture. Then, 5 ml of themixture was dropped into the above-mentioned heated dispersion at a rateof 5 mL/h (at 0.5 mmol/h based on In). When the addition of the mixtureis finished, the heating source was turned off, allowing the solutioncontaining the dispersion into which the mixture is added to cool downto the room temperature. Subsequently, methanol was added to thesolution, so that precipitates of the core-shell semiconductornanoparticles were obtained. Then, solid components were collected fromthe solution by centrifugal separation (with a radius of 150 mm at 2,500rpm for 3 minutes). The solid components were dissolved into chloroform,and thereafter various properties of the obtained core-shellsemiconductor nanoparticles were measured. Upon measuring an averageparticle size of particles covered with the shell, the average particlesize of the core-shell semiconductor nanoparticles was 11.4 nm and thatan average thickness of the shell was approximately 3.0 nm based on adifference in the average particle size between the primarysemiconductor nanoparticles and the core-shell semiconductornanoparticles.

(3) Measurement of Absorption, Emission, and Excitation Spectra

The photoluminescence, absorption, and excitation spectra were measured.The results are shown in FIGS. 2 and 3. The photoluminescence spectrumwas measured at an excitation wavelength of 500 nm using a multi-channelspectrometer (manufactured by HAMAMATSU PHOTONICS K.K., trade name:PMA12). The absorption spectrum was measured at wavelengths of 350 nm to850 nm using an ultraviolet-visible-near-infrared spectrophotometer(manufactured by JASCO Corporation, trade name: V-670). The excitationspectrum was measured at observation wavelengths of 593 nm and 780 nm(i.e., corresponding to peaks observed in the photoluminescencespectrum) using a fluorescence spectrophotometer (manufactured byHORIBA, Ltd., trade name: Fluoromax-4). As shown in FIG. 2, thecore-shell semiconductor nanoparticles obtained in Example 1 wereobserved to exhibit a sharp photoluminescence peak at a wavelength ofaround 593 nm with a full width at half maximum of approximately 38 nm.As shown in FIG. 3, in the excitation spectrum at the observationwavelength of 593 nm, an exciton peak was observed at a wavelength ofaround 540 nm. For a reference, FIGS. 4 and 5 show a photoluminescencespectrum and an absorption spectrum of the primary semiconductornanoparticles (Ag—In—S). Ag—In—S not covered with a shell exhibited abroad photoluminescence spectrum with a peak at around 820 nm, while theabsorption and excitation spectrum had no exciton peak.

In Example 1, the photoluminescence lifetime of photoluminescenceobserved to be the sharp peak was measured. In measurement of thephotoluminescence lifetime, using a fluorescence lifetime measurementdevice (trade name: Quantaurus-Tau) manufactured by HAMAMATSU PHOTONICSK.K, the core-shell semiconductor nanoparticles were irradiated withlight having a wavelength of 470 nm as the excitation light to measure adecay curve of the photoluminescence at around the peak wavelength ofthe sharp photoluminescence peak. The obtained decay curve was dividedinto three components by parameter fitting using a fluorescence lifetimemeasurement/analysis software U11487-01, manufactured by HAMAMATSUPHOTONICS K.K. As a result, τ₁, τ₂, and τ₃, and contribution rates ofrespective components (A₁, A₂, and A₃) were determined as shown in Table1 below.

TABLE 1 τ₁ A₁ τ₂ A₂ τ₃ A₃ (ns) (%) (ns) (%) (ns) (%) Example 1 3.1 55.728.0 34.5 121.7 9.8 Reference Example 919.0 1 — — — — (Ag—In—S)

As shown in Table 1, a main component (τ3, A3) had a lifetime of 121.7ns. Meanwhile, a component (τ2, A2) having a photoluminescence lifetimeof 28.0 ns was also observed at an intensity similar to that of the maincomponent. This photoluminescence lifetime was similar to aphotoluminescence lifetime (30 ns to 60 ns) of the component with thelargest contribution rate in photoluminescence emitted from CdSe(nanoparticles) in which band-edge emission was confirmed.

Example 2

First, 10 nmol of the primary semiconductor nanoparticles, obtained inthe same procedure as in Example 1, at amount of nanoparticles wasdispersed into 12 mL of oleylamine to obtain a dispersion, which wasthen held at 260° C. under an Ar atmosphere. Separately from this,gallium acetylacetonate (Ga(acac)₃) and sulfur powder were added tooleylamine at concentrations of 0.1 mmol/mL and 0.15 mmol/mL,respectively, and then stirred at 100° C. for 10 minutes, so that thegallium acetylacetonate and sulfur powder were dissolved and a mixturewas obtained. Then, 4 ml of the mixture was dropped into theabove-mentioned heated dispersion at a rate of 5 mL/h (at 0.5 mmol/hbased on Ga). When the addition of the mixture is finished, the heatingsource was turned off, allowing the solution containing the dispersionwith the mixture to cool down to the room temperature. Thereafter,methanol was added to the solution, so that precipitates of thecore-shell semiconductor nanoparticles is obtained. Then, solidcomponents were collected from the solution, by centrifugal separation(with a radius of 150 mm at 2,500 rpm for 3 minutes). The solidcomponents were dissolved into chloroform, and thereafter variousproperties of the obtained core-shell semiconductor nanoparticles weremeasured. Upon measuring an average particle size of particles coveredwith the shell, the average particle size of the core-shellsemiconductor nanoparticles was 6.65 nm. The average particle size ofthe primary semiconductor nanoparticles produced in this example was5.99 nm. An average thickness of the shell was determined to beapproximately 0.33 nm on average based on a difference in the averageparticle size between the primary semiconductor nanoparticles and thecore-shell semiconductor nanoparticles.

Although the primary semiconductor nanoparticles were produced in thesimilar procedure as in Example 1, the average particle size of theprimary semiconductor nanoparticles in Example 2 differs from that ofthe primary semiconductor nanoparticles produced in Example 1. It isassumed that this is because a period of time after mixing the reagentsbefore starting a heating process in Example 2 differs slightly fromthat in Example 1, which allows for varying the amount of substance of asilver-thiol complex that is expected to be formed during that time. Thesame goes for the examples in the below and the like.

The photoluminescence spectrum (at the excitation wavelength of 450 nm),the absorption spectrum, and the excitation spectra (at the observationwavelengths of 580 nm and 730 nm) were measured using the same devicesas those used in Example 1. The results are shown in FIGS. 6 and 7. Asshown in FIG. 6, the core-shell semiconductor nanoparticles obtained inExample 2 were observed to exhibit a sharp emission peak at a wavelengthof around 584 nm with a full width at half maximum of approximately 39nm. As shown in FIG. 7, in the excitation spectrum at the observationwavelength of 580 nm, an exciton peak was observed at a wavelength ofaround 530 nm. A photoluminescence quantum yield at the sharp peak (at awavelength of 530 to 650 nm), which was assumed to be the band-edgeemission, was measured using a multichannel spectrometer (at anexcitation wavelength of 450 nm), which was of the same type as thatused in Example 1. As a result, the luminescent quantum yield was 1.9%.Furthermore, the photoluminescence lifetime of photoluminescenceobserved as the sharp peak were measured in similar procedure as that inExample 1. Consequently, τ₁, τ₂, and τ₃, and contribution rates ofrespective components (A₁, A₂, and A₃) were determined as shown in Table2 below. As shown in Table 2, the photoluminescence lifetime of the maincomponent (τ₂, A₂) was 39.1 ns.

TABLE 2 τ₁ A₁ τ₂ A₂ τ₃ A₃ (ns) (%) (ns) (%) (ns) (%) Example 2 5.8 53.139.1 38.4 173.3 8.5

Example 3

First, 30 nmol of the primary semiconductor nanoparticles, obtained inthe same procedure as in Example 1, at amount of nanoparticles, 0.1 mmolof gallium acetylacetonate (Ga(acac)₃), 0.05 mmol of dibenzyldisulfideand 12 mL of oleylamine were charged into a flask and then heated to260° C. at a temperature increasing rate of 10° C./min. After thesolution was held at 260° C. for 20 minutes, the heating source wasturned off, allowing the solution to cool. Thereafter, methanol wasadded to the solution, so that precipitates of the core-shellsemiconductor nanoparticles is obtained. Then, solid components werecollected from the solution by centrifugal separation (with a radius of150 mm at 2,500 rpm for 3 minutes). The solid components were dissolvedinto chloroform, and then various properties of the obtained core-shellsemiconductor nanoparticles were measured. Upon measuring an averageparticle size of particles each covered with the shell, the averageparticle size of the core-shell semiconductor nanoparticles was 8.0 nm.The average particle size of the primary semiconductor nanoparticlesproduced in Example 2 was 6.0 nm. Thus, an average thickness of theshell was approximately 1 nm based on a difference in the averageparticle size between the primary semiconductor nanoparticles and thecore-shell semiconductor nanoparticles.

The photoluminescence spectrum (at the excitation wavelength of 450 nm),the absorption spectrum, and the excitation spectra were measured usingthe same devices as those used in Example 1. The results are shown inFIGS. 8 and 9. As shown in FIG. 8, the core-shell semiconductornanoparticles obtained in Example 3 were observed to exhibit a sharpphotoluminescence peak at a wavelength of around 586 nm with a fullwidth at half maximum of approximately 39 nm. As shown in FIG. 9, in theexcitation spectrum at the observation wavelength of 585 nm, an excitonpeak was observed at a wavelength of around 535 nm. Thephotoluminescence quantum yield of the core-shell semiconductornanoparticles were measured in the same way as in Example 2. Thephotoluminescence quantum yield was 14.3%. Furthermore, thephotoluminescence lifetime of photoluminescence observed as the sharppeak were measured in the same procedure as that in Example 1.Consequently, τ₁, τ₂, and τ₃, and contribution rates of respectivecomponents (A₁, A₂, and A₃) were determined as shown in Table 3 below.As shown in Table 3, the photoluminescence lifetime of the maincomponent (τ₂, A₂) was 51.4 ns.

TABLE 3 τ₁ A₁ τ₂ A₂ τ₃ A₃ (ns) (%) (ns) (%) (ns) (%) Example 3 15.1 1851.4 76 218.5 5

When comparing the photoluminescence spectra between the semiconductornanoparticles obtained in Example 1 and Example 2, the photoluminescenceintensity of the sharp peak (i.e., intensity of the band-edge emission)in Example 2 was definitely higher than the photoluminescence intensityof a broad peak (i.e., intensity of defective photoluminescence). Thisshows that the formation of the shell by GaS_(x) has the advantage overthe formation of the shell by InS_(x) in obtaining the band-edgeemission from the core.

When comparing the photoluminescence quantum yield of the semiconductornanoparticles obtained in Example 2 and that in Example 3, thephotoluminescence quantum yield in Example 3 was confirmed to be 7.5times higher than that in Example 2. In the formation of the shell, thethickness of the shell in Example 3 was increased, more specifically, tobe approximately 1 nm, even though Examples 2 and 3 had the same molarratio of the Ga source for formation of the shell to the primarysemiconductor nanoparticles. In view of these, it is assumed that theuse of dibenzyldisulfide as a sulfur source allows the shell tocontinuously cover the surface of the core, which allows for effectivelyremoving the defects on the surface of the core, so that intensity ofthe band-edge emission is increased.

Comparative Example 1

First, 10 nmol of the primary semiconductor nanoparticles, obtained inthe same procedure as in Example 1, at amount of nanoparticles wasdispersed into 12 mL of oleylamine to obtain a dispersion, which wasthen held at 260° C. under an Ar atmosphere. Separately from this, zincacetate (Zn(OAc)₂) and sulfur powder were added to oleylamine each at aconcentration of 0.1 mmol/mL, and then heated at 100° C. for 10 minutes,so that the zinc acetate and the sulfur powder were dissolved to producea mixture. Then, 2 ml of the mixture was dropped into theabove-mentioned heated dispersion at a rate of 5 mL/h (at 0.5 mmol/h interms of Zn). When the addition of the mixture was finished, the heatingsource was turned off, allowing the solution containing the dispersioninto which the mixture was added to cool down to the room temperature.Thereafter, methanol was added to the solution, so that producingprecipitates of the core-shell semiconductor nanoparticles. Then, solidcomponents were collected from the solution by centrifugal separation(with a radius of 150 mm at 2,500 rpm for 3 minutes). The obtained solidcomponents were dissolved in chloroform, and then the photoluminescencespectrum (at the excitation wavelength of 450 nm) and the absorptionspectrum were measured by using the same devices as those used inExample 1. The results are shown in FIGS. 10 and 11.

As shown in FIG. 10, with the nanoparticles obtained in ComparativeExample 1, any sharp photoluminescence peak as in Examples 1 to 3 werenot observed, and a broad photoluminescence spectrum were observed. Thatis, no band-edge emission was observed, although ZnS has a bandgapenergy larger than that of Ag—In—S and can exhibit the type-1 bandalignment. It is assumed that this is because a solid solution wasformed between the core and shell, so that the core-shell structure wasnot formed.

Examples 4A to 4D

First, 30 nmol of the primary semiconductor nanoparticles, obtained inthe same procedure as in Example 1, at amount of nanoparticles, 0.1 mmolof gallium acetylacetonate (Ga(acac)₃), 0.05 mmol of dibenzyldisulfideand 12 mL of oleylamine were charged into a flask and then heated to260° C. at a temperature increasing rate of 10° C./min. Immediatelyafter the solution reaches 260° C. (Example 4A), or alternatively, afterthe solution was held at 260° C. for 10 minutes (Example 4B), for 20minutes (Example 4C), and for 30 minutes (Example 4D), then the heatingsource was turned off to allow the solution to cool. Thereafter, eachliquid containing a reaction product was subjected to centrifugalseparation (with a radius of 150 mm at 2,500 rpm for 10 minutes), and asupernatant was taken out. Then, methanol was added to the supernatantsolution until nanoparticles were precipitated, followed by centrifugalseparation (with a radius of 150 mm at 2,500 rpm for 5 minutes), andsolid components were collected. The solid components were dissolvedinto chloroform, and then various properties of the obtained core-shellsemiconductor nanoparticles were measured.

Examples 5A to 5F

First, 30 nmol of the primary semiconductor nanoparticles, obtained inthe same procedure as in Example 1, at amount of nanoparticles, 0.1 mmolof gallium acetylacetonate (Ga(acac)₃), 0.15 mmol of sulfur, and 12 mLof oleylamine were charged into a flask and then heated to 260° C. at atemperature increasing rate of 10° C./min. Immediately after thesolution reaches 260° C. (Example 5A), or alternatively, after thesolution was held at 260° C. for 10 minutes (Example 5B), for 30 minutes(Example 5C), for 40 minutes (Example 5D), for 50 minutes (Example 5E),or for 60 minutes (Example 5F), the heating source was turned off toallow the solution to cool. Thereafter, each liquid containing areaction product was subjected to centrifugal separation (with a radiusof 150 mm at 2,500 rpm for 10 minutes), and a supernatant was taken out.Then, methanol was added to the supernatant until nanoparticles wereprecipitated, followed by centrifugal separation (with a radius of 150mm at 2,500 rpm for 5 minutes), and solid components were collected. Thesolid components were dissolved into chloroform, and then variousproperties of the obtained core-shell semiconductor nanoparticles weremeasured.

The photoluminescence spectra (at the excitation wavelength of 400 nm)of the semiconductor nanoparticles obtained in Examples 4A to 4D and 5Ato 5F were measured using the same device as that used in Example 1. Thephotoluminescence spectra are shown in FIG. 12 (Examples 4A to 4D) andin FIG. 13 (Examples 5A to 5F). In each diagram, the photoluminescencespectrum of the primary semiconductor nanoparticles (i.e., core) nothaving a shell is also shown as a reference example.

As shown in FIG. 12, among Examples 4A to 4C in which dibenzyldisulfidewas used as the sulfur source to form the shell, Example 4C (with atemperature holding time of 20 minutes) had the highest peak intensityof the band-edge emission. Example 4D (with a temperature holding timeof 30 minutes) exhibited a peak intensity of the band-edge emissionlower than that in Example 4C. This shows that forming the shell usingdibenzyldisulfide with a relatively short temperature holding timeallows for increasing band-edge emission intensity. This can also beconfirmed by FIG. 14, which shows the relationship between the peakintensity of the band-edge emission and the temperature holding time ineach of Examples 4A to 4C.

As shown in FIG. 13, among Examples 5A to 5F in which an elementalsulfur was used as the sulfur source to form the shell, Example 5E (witha temperature holding time of 50 minutes) had the highest peak intensityof the band-edge emission. This shows that, in the case of forming theshell using the elemental sulfur as the sulfur source, a longertemperature holding time is required to obtain the core-shell particlesthat exhibit the higher peak intensity of the band-edge emission. Thiscan also be confirmed by FIG. 14, which shows the relationship betweenthe peak intensities of the band-edge emission and the temperatureholding times in Examples 5A to 5F. As illustrated in FIG. 14, formingthe shell using elemental sulfur with a longer temperature holding timeallows for achieving the peak intensity of the band-edge emission thatis higher than that of the maximum peak intensity of the band-edgeemission achieved in the case of using dibenzyldisulfide.

Furthermore, as shown in FIG. 13, forming the shell using the elementalsulfur with a longer temperature holding time allows the intensity ofthe band-edge emission (i.e., the ratio of the band-edge emissionintensity/defective photoluminescence intensity) to be increased (inparticular, Examples 5E and 5F).

FIG. 15 is a graph showing the relationship between the peak intensitiesof the band-edge emission and temperature holding times in Examples 4Bto 4D and Examples 5B to 5F. It is found that, in each case of usingdibenzyldisulfide or elemental sulfur, the peak wavelength is shiftedtoward the longer wavelength side as the temperature holding time isincreased.

Observation with HRTEM and HAADF

The core-shell semiconductor nanoparticles in Example 5F were observedwith HRTEM and HAADF (manufactured by JEOL Ltd., trade name: ARM-200F).FIG. 16 shows an HRTEM image, and FIG. 17 shows an HAADF image. In eachimage, a crystal core with a regular pattern and a shell surrounding thecore and having no regular pattern were observed, and the shell wasobserved to be amorphous in the semiconductor nanoparticles of Example5F.

Analysis with XPS

The core-shell semiconductor nanoparticles in Example 5F were analyzedby an X-ray photoelectron spectroscopy system (manufactured by SHIMADZUCorporation, trade name KRATOS AXIS-165X). When the peak positionsderived from 3d orbital of silver and indium were compared withdatabase, it was confirmed to be identical to the positions of theirsulfides. When the peak position derived from 2p orbital of sulfur wascompared with database, it was confirmed to be identical to the positionof a metal sulfide. Regarding gallium, no database on its sulfide wasavailable, so that whether gallium was present in the form of sulfidewas not able to be confirmed. However, the peak position derived from 3dorbital of gallium obviously differed from that of the metal gallium,and was positioned between the peak of gallium oxide and the peak ofgallium selenide. Thus, gallium is highly likely to be present in theform of sulfide.

Analysis with Energy Dispersive X-Ray Analyzer

An atomic percentage of each element contained in the core-shellsemiconductor nanoparticles in Example 5F was analyzed with an energydispersive X-ray analyzer (manufactured by EDAX Inc., trade name:OCTANE). The result is shown in Table 4. Assuming that the compositionof the core-shell semiconductor nanoparticles is AgInS₂.Ga₂S₃, an atomicpercentage of sulfur was calculated from the result of Ag and Ga shownin Table 4 and determined to be 53.9 (17.2×2+13.1÷2×3=53.9), which showsgood agreement with the value of S shown in Table 4.

TABLE 4 Ag In Ga S Example 5F 17.2% 17.0% 13.1% 52.7%

Example 6 (1) Production of Primary Semiconductor Nanoparticles(Ag—In—S)

First, 0.4 mmol of each of silver acetate (AgOAc) and indium acetate(In(OAc)₃), 0.8 mmol of thiourea, 35.8 mmol of n-tetradecylamine, and0.2 mL of 1-dodecanethiol were introduced into a two-necked flask andsubjected to vacuum deaeration (for 3 minutes at normal temperature),followed by heating under an Ar atmosphere at a temperature increasingrate of 5° C./min up to 150° C. The obtained suspension was allowed tocool, followed by addition of a small amount of hexane thereto at atemperature just below 90° C. Then, the suspension was subjected tocentrifugal separation (with a radius of 150 mm at 4,000 rpm for 10minutes), and a supernatant dark red solution was taken out. Then,methanol was added to the solution until nanoparticles wereprecipitated, followed by centrifugal separation (with a radius of 150mm at 2,500 rpm for 3 minutes). Then, the precipitates were dried at thenormal temperature under vacuum, so that semiconductor nanoparticles(primary semiconductor nanoparticles) were obtained.

(2) Covering with Shell

First, 30 nmol of the primary semiconductor nanoparticles as the core,which were obtained in the step (1), at amount of nanoparticles, 0.1mmol of gallium acetylacetonate (Ga(acac)₃), 0.15 mmol of sulfur, and36.48 mmol of n-tetradecylamine were charged into a flask and thenheated to 260° C. at a temperature increasing rate of 10° C./min. Afterthe solution was held at 260° C. for 50 minutes, the heating source wasturned off to allow the solution to cool. Thereafter, methanol was addedto the solution, thereby producing precipitates of the core-shellsemiconductor nanoparticles. Then, solid components were collected fromthe solution by centrifugal separation (with a radius of 150 mm at 2,500rpm for 3 minutes). The solid components were dissolved into chloroform,and then various properties of the obtained core-shell semiconductornanoparticles were measured. Upon measuring an average particle size ofparticles covered with the shell, the average particle size of thecore-shell semiconductor nanoparticles was 8.6 nm. The average particlesize of the primary semiconductor nanoparticles produced in this examplewas 7.1 nm. An average thickness of the shell was determined to beapproximately 0.75 nm based on a difference in the average particle sizebetween the primary semiconductor nanoparticles and the core-shellsemiconductor nanoparticles.

The photoluminescence spectrum (at the excitation wavelength of 450 nm)and the absorption spectrum were measured using the same devices asthose used in Example 1. The results are shown in FIGS. 18 and 19. Asshown in FIG. 18, the core-shell semiconductor nanoparticles obtained inExample 6 were observed to exhibit a sharp photoluminescence peak at awavelength of around 581 nm with a full width at half maximum ofapproximately 28 nm. As shown in FIG. 19, in the excitation spectrum atthe observation wavelength of 585 nm, an exciton peak was observed at awavelength of around 535 nm. A photoluminescence quantum yield at thesharp peak (at a wavelength of 530 to 650 nm), which was assumed to bethe band-edge emission, was measured by using a multichannelspectrometer (at an excitation wavelength of 450 nm), which was of thesame type as that used in Example 1. As a result, the photoluminescencequantum yield was 21.1%.

The embodiments of the present disclosure provide the semiconductornanoparticles that enable the band-edge emission and can be used as awavelength conversion material of a light-emitting device or as abiomolecule marker.

Although the present disclosure has been described with reference toseveral exemplary embodiments, it shall be understood that the wordsthat have been used are words of description and illustration, ratherthan words of limitation. Changes may be made within the purview of theappended claims, as presently stated and as amended, without departingfrom the scope and spirit of the disclosure in its aspects. Although thedisclosure has been described with reference to particular examples,means, and embodiments, the disclosure may be not intended to be limitedto the particulars disclosed; rather the disclosure extends to allfunctionally equivalent structures, methods, and uses such as are withinthe scope of the appended claims.

One or more examples or embodiments of the disclosure may be referred toherein, individually and/or collectively, by the term “disclosure”merely for convenience and without intending to voluntarily limit thescope of this application to any particular disclosure or inventiveconcept. Moreover, although specific examples and embodiments have beenillustrated and described herein, it should be appreciated that anysubsequent arrangement designed to achieve the same or similar purposemay be substituted for the specific examples or embodiments shown. Thisdisclosure may be intended to cover any and all subsequent adaptationsor variations of various examples and embodiments. Combinations of theabove examples and embodiments, and other examples and embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the description.

In addition, in the foregoing Detailed Description, various features maybe grouped together or described in a single embodiment for the purposeof streamlining the disclosure. This disclosure may be not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive subject matter may bedirected to less than all of the features of any of the disclosedembodiments. Thus, the following claims are incorporated into theDetailed Description, with each claim standing on its own as definingseparately claimed subject matter.

The above disclosed subject matter shall be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments which fall within thetrue spirit and scope of the present disclosure. Thus, to the maximumextent allowed by law, the scope of the present disclosure may bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

What is claimed is:
 1. A semiconductor nanoparticle comprising: a corecomprising a semiconductor being adapted to emit photoluminescence uponbeing irradiated with light; and a shell covering a surface of the coreand having a bandgap energy larger than a bandgap energy of the core,the shell being in heterojunction with the core, wherein thesemiconductor contains M¹, M², and Z, wherein M¹ is at least one elementselected from the group consisting of Ag, Cu, and Au, and comprises atleast Ag, M² is at least one element selected from the group consistingof Al, Ga, In, and Tl, and comprises at least In, and Z is at least oneelement selected from the group consisting of S, Se, and Te, andcomprises at least S, wherein the shell comprises a semiconductorcontaining a Group 13 element and a Group 16 element, and wherein aphotoluminescence lifetime of the semiconductor nanoparticles is 200 nsor less.
 2. The semiconductor nanoparticle according to claim 1, whereinthe shell contains In as the Group 13 element.
 3. The semiconductornanoparticle according to claim 2, wherein the shell contains S as theGroup 16 element.
 4. The semiconductor nanoparticle according to claim1, wherein the shell contains Ga as the Group 13 element.
 5. Thesemiconductor nanoparticle according to claim 4, wherein the shellcontains S as the Group 16 element.
 6. The semiconductor nanoparticleaccording to claim 1, wherein the core contains Ag as M¹, In as M², andS as Z.
 7. The semiconductor nanoparticle according to claim 1, whereinthe core further contains M³, wherein M³ is at least one elementselected from the group consisting of Zn and Cd.
 8. The semiconductornanoparticle according to claim 1, wherein a photoluminescence spectrumof the semiconductor nanoparticle has a peak with a full width at halfmaximum of 50 nm or less.
 9. The semiconductor nanoparticle according toclaim 1, wherein the semiconductor nanoparticle has an average particlesize of from 1 nm to 20 nm.
 10. The semiconductor nanoparticle accordingto claim 1, wherein the core has an average particle size of from 2 nmto 10 nm.
 11. The semiconductor nanoparticle according to claim 1,wherein a thickness of the shell is in a range of 0.1 nm to 20 nm. 12.The semiconductor nanoparticle according to claim 1, wherein anexcitation spectrum or an absorption spectrum of the semiconductornanoparticles exhibits an exciton peak.
 13. A light-emitting devicecomprising a light conversion member and a semiconductor light-emittingelement, wherein the light conversion member contains a plurality of thesemiconductor nanoparticle according to claim
 1. 14. The light-emittingdevice according to claim 13, wherein the semiconductor light-emittingelement is a light-emitting diode chip.
 15. A method of producingsemiconductor nanoparticles, comprising: providing a dispersion bydispersing primary semiconductor nanoparticles into a solvent, each ofthe primary semiconductor nanoparticles comprising a semiconductor thatcontains M¹, M², and Z, wherein M¹ is at least one element selected fromthe group consisting of Ag, Cu, and Au, and comprises at least Ag, M² isat least one element selected from the group consisting of Al, Ga, In,and Tl, and comprises at least In, and Z is at least one elementselected from the group consisting of S, Se, and Te, and comprises atleast S; and adding, to the dispersion, a compound containing a Group 13element, and an elemental substance of a Group 16 element or a compoundcontaining the Group 16 element to form a semiconductor layer comprisingthe Group 13 element and the Group 16 element on a surface of each ofthe primary semiconductor nanoparticles, wherein a photoluminescencelifetime of the semiconductor nanoparticles is 200 ns or less.
 16. Themethod of producing semiconductor nanoparticles according to claim 15,wherein the compound containing the Group 16 element isdibenzyldisulfide.
 17. The method of producing semiconductornanoparticles according to claim 15, further comprising, after theadding: heating the dispersion at a predetermined rate up to a peaktemperature that is in a range of 200° C. to 290° C., and holding thedispersion at the peak temperature for a predetermined period of time.18. The method of producing semiconductor nanoparticles according toclaim 17, wherein the elemental substance of the Group 16 element is anelemental sulfur, and the predetermined period of time for holding thedispersion at the peak temperature is not less than 40 minutes and notmore than 60 minutes.
 19. The method of producing semiconductornanoparticles according to claim 15, wherein the dispersion containsn-tetradecylamine.